The present invention relates to a secondary battery and, more particularly, a safety system of a secondary battery which can reduce internal pressure thereof to prevent the explosion of the secondary battery.
Generally, a secondary battery is a rechargeable battery such as a nickel-metal hydride battery, a lithium battery or a lithium-ion battery which is widely used in many applications. Such a secondary battery is subject to high internal pressures due to gases generated by chemical reactions when it is discharged and recharged. Though not common, it is possible for the battery to explode as a result of the gases produced.
Therefore, many presently known secondary batteries have a cap assembly provided with a safety valve which can discharge gases through a discharge hole formed on a cap cover so as to reduce the pressure in the batteries when the pressure in the battery is excessively raised.
The lithium-ion battery comprises an electrode assembly inserted into a cap. A cap assembly is mounted on an upper end of the can. Electrolyte is injected into the can through an inlet port formed on the cap assembly. Insulating members are disposed between the electrode assembly and the can.
Referring to FIG. 3, there is shown a conventional cap assembly 2. The cap assembly 2 comprises a negative portion 4 welded on an upper end of the can, a positive portion 6 disposed on a central portion of the negative portion 4, and an insulating plate 8 disposed between the negative portion 4 and positive portion 6. A rivet 10 penetrates through the negative potion 4 and positive portion 6 is coupled to the positive electrode of the roll electrode assembly.
In the above described conventional lithium-ion battery, if the pressure within the battery is abruptly increased by the gas generated therein, the battery may explode. Thus, there is provided gas release means in the secondary battery.
That is, safety grooves 12 are formed in the negative portion 4 of the cap assembly 2 through a mechanical process, etching or electroforming process. The safety grooves 12 are broken open when the internal pressure of the battery is increased above a predetermined level, thereby preventing the battery from exploding.
An electrolyte injection hole 14 is formed on the negative portion 4 of the cap assembly 2. After the electrolyte is injected through the hole 14, a plug 16 is snugly fitted into the hole 14 and is then welded to provide a seal.
FIG. 4 shows another example of a conventional cap assembly 2.
In this example, a ball(not shown) is inserted into the injection hole 14 and is then welded to provide a seal.
In the above described conventional cap assembly shown in FIGS. 3 and 4, since the safety grooves and the electrolyte injection hole are formed on the negative portion which is small in size, it is difficult to design and manufacture the same.
In the conventional cap assembly shown in FIG. 4, since the diameter of the injection hole is small, it is difficult to inject electrolyte into the can.
Therefore, the present invention has been made in an effort to solve the above described drawbacks of the prior art.
It is an object of the present invention to provide a cap assembly for a secondary battery, in which safety means can precisely operate at a pre-set pressure to prevent the battery from exploding.
It is another object of the present invention to provide a cap assembly in which the safety means can be easily formed, thereby reducing manufacturing costs.
To achieve the above objects, the present invention provide a secondary battery comprising a can into which an electrode assembly is inserted; a cap assembly mounted to an opening of the can; and an electrolyte injection hole formed on at least one of the can and cap; and a safety member mounted to tightly close the electrolyte injection hole.
Preferably, the electrolyte injection hole is formed on a negative portion of the cap assembly.
Preferably, the safety member is inserted into and welded on the electrolyte injection hole.
Alternatively, the safety member is disposed on the electrolyte injection hole and is welded on the negative portion.
Preferably, the safety member is made of a material selected from the group consisting of aluminum, nickel, stainless steel, and nickel gilding steel plate.
Preferably, the safety member is designed to be broken by battery of about 10 to 30 kgf/cm2.
Alternatively, the safety member is provided with a plurality of grooves.